The genus Shigella includes four species (major serogroups): S. dysenteriae (Grp. A), S. flexneri (Grp. B), S. boydii (Grp. C) and S. sonnei (Grp. D) as classified in Bergey's Manual for Systematic Bacteriology (N. R. Krieg, ed., pp. 423-427 (1984)). These serogroups are further subdivided into serotypes (Table 1). The genera Shigella and Escherichia are phylogenetically closely related. Brenner and others have suggested that the two are more correctly considered sibling species based on DNA/DNA reassociation studies (D. J. Brenner, et al., International J. Systematic Bacteriology, 23:1-7 (1973)). These studies showed that Shigella species are on average 80-89% related to E. coli at the DNA level. Also, the degree of relatedness between Shigella species is on average 80-89%. Shigella boydii serotype 13 is atypical in that it is only 65% related to other Shigella serotypes and Escherichia.
The genus Shigella is pathogenic in humans; it causes dysentery at levels of infection of 10 to 100 organisms. By contrast, the majority of E. coli (9000 O:H serotypes) are not associated with diarrheal disease. Pathogenic E. coli serotypes are collectively referred to as Enterovirulent E. coli (EVEC) (J. R. Lupski, et al., J. Infectious Diseases, 157:1120-1123 (1988); M. M. Levine, J. Infectious Diseases, 155:377-389 (1987); M. A. Karmali, Clinical Microbiology Reviews, 2:15-38 (1989)). This group includes at least 5 subclasses of E. coli, each having a characteristic pathogenesis pathway resulting in diarrheal disease. The subclasses include Enterotoxigenic E. coli (ETEC), Verotoxin-Producing E. coli (VTEC), Enteropathogenic E. coli (EPEC), Enteroadherent E. coli (EAEC) and Enteroinvasive E. coli (EIEC). The VTEC include Enterohemorrhagic E. coli (EHEC) since these produce verotoxins.
The pathogenesis of Enteroinvasive E. coli is very similar to that of Shigella. In both, dysentery results from invasion of the colonic epithelial cells followed by intracellular multiplication which leads to bloody, mucous discharge with scanty diarrhea.
Thus, detection of Shigella and EIEC is important in various medical contexts. For example, the presence of either Shigella or EIEC in stool samples is indicative of gastroenteritis, and the ability to screen for their presence is useful in treating and controlling that disease. Detection of Shigella or EIEC in any possible transmission vehicle such as food is also important to avoid spread of gastroenteritis.
Currently, presence of Shigella in stool samples is detected by cultivating an appropriately prepared sample on microbiological media under conditions favorable for growth of those bacteria. The resulting colonies are then examined for microbiological and biochemical characteristics, a process that typically takes at least three days and does not permit processing large numbers of samples. However, hospitals do not test for the presence of EIEC in stool because of the difficulty of serotyping which is necessary to identify the EIEC among the numerous, non-pathogenic E. coli normally present in stool.